An AC electric metering system planned for extensive future use by electric utility companies, employs magneto-optical current transformers, hereinafter called MOCTs, for sensing current in extra high voltage transmission lines. Such MOCTs have been found to offer certain advantages over conventional current transformers that utilize copper and iron designs. For instance, the MOCT enjoys total electric isolation and therefore does not require the additional costly insulating techniques that a conventional current transformer requires. Additionally, the MOCT enjoys immunity to electro-magnetic radiation that is not similarly enjoyed by conventional current transformers. For these reasons and various other reasons known to those skilled in this technology, the MOCT is being contemplated for more widespread usage in the electrical utility industry. An example of an AC electric metering system employing an MOCT is found in U.S. Pat. No. 4,578,639 which issued on Mar. 26, 1986 to Robert C. Miller. Various other developments relating to this technology have occured recently, an example of which is U.S. Pat. No. 4,540,937 which issued to Juris A. Asars on June 7, 1983 wherein it is disclosed that an electronic circuit compensates for the DC component of a substantially AC output signal when a magneto-optical current sensor using the Faraday effect is utilized.
One of the key elements making the use of MOCTs favorable, is the Faraday rotator which employs the principal of the Faraday effect. It is known that the Faraday effect establishes that when a plane polarized beam of light passes through a certain transparent substance having known optical characteristics, the plane of the polarized light emitted from the transparent substance is different than that which entered; this change in planar orientation being determined as a function of the magnetic field by which the transparent substance is disposed. It is known that by measuring the change in the planar orientation, the magnitude of the current flowing in the transmission line which passes through the Faraday rotator can be determined.
The Faraday rotator type optical sensor can also be used in other applications for which it is desired to sense current flow through a load. Besides the application for MOCTs, the Faraday rotator has been used in electric motor and transformer arrangement to detect an overload current condition in a high voltage electrical load and to transmit this information to a low voltage processing circuit which provides desired isolation between the high voltage load and the low voltage processing circuit. This application of a Faraday rotator can be seen in U.S. Pat. No. 4,363,061 issued on Dec. 7, 1982 to Eugene G. Vaerewyck et al.
In applying the Faraday rotator to an AC metering situation, it is required that the method by which the beam of light is introduced to the transparent substance contribute as little attenuation of the optical signal as possible. It is further required that this interfacing arrangement not introduce additional signal losses for other reasons such as because of spherical aberrations or surface contamination. Examples of the current method of connecting the fiber optic cable to a Faraday rotator can be found in the previously referenced U.S. Pat. No. 4,578,639 as well as Japanese Patent No. 58-153174 which issued on Sept. 12, 1983 to Tadashi Satou. These references illustrate a typical collimator/lens arrangement for connecting each of the input and output optical cables to the Faraday rotator. In these arrangements, a simple lens is used to collect the light emanating from the end of the input fiber optic cable, and a collimator element then collimates this light into a bundle of parallel light which passes through the Faraday rotator. Associated with this technique is a total of eight glass-to-air interfaces which, unless the surfaces are coated, results in reflection loss of approximately 0.17 db per interface for a total attenuation of approximately 1.4 db. It is understood that these eight interfaces are easily contaminated with dirt or condensed moisture causing additional, possibly large, increases in attenuation. Additionally, because existing interface arrangements utilize a spherically shaped lens, it has been found that several additional dbs attenuation is introduced due to spherical aberration.
Further detrimental to the use of a spherical lens is the inherent disadvantage that a spherical lens is not achromatic; that is, the spherical lens will work at only one wave length. One can understand the severity of this characteristic when one tries to assemble and install such a collimator/lens arrangement for system use. On assembling this lens arrangement, it is ultimately desired to receive at the output fiber optic cable as large a signal as possible relative to the magnitude of the optic signal injected into the input fiber optic cable. To verify that this in fact occurs, a signal in the infrared wavelength must be used since this is the signal under which the device will operate. As such, in order to set up the Faraday rotator using this type of collimator/lens arrangement, special testing and tooling is required to verify the level of the infrared signal input and the magnitude of the infrared signal output from the respective fiber optic cables.
Examples of prior art patents which also disclose this type of collimator/lens arrangement for application in AC metering systems are U.S. Pat. No. 4,428,017 issued on Jan. 24, 1984 to Eugene G. Vaerewyck et al. and U.S. Pat. No. 4,612,500 issued to Chen et al. on Sept. 16, 1986.
Further examples of U.S. patents disclosing alternate arrangements for communicating optical signals and connecting fiber optic cables are U.S. Pat. No. 4,445,751 issued on May 1, 1984 to William C. Divens et al., U.S. Pat. No. 4,474,429 issued on Oct. 2, 1984 to Bulent E. Yoldas et al. and U.S. Pat. No. 4,613,811 issued on Sept. 23, 1986 to Eugene G. Vaerewyck et al. The reference U.S. Pat. No. 4,445,751 provides an interferometer system which uses titanium to form an optical waveguide; reference U.S. Pat. No. 4,474,429 provides an ion-polished optical cable coated with a liquid that contains glass constituents that can be fused to form an optical connection; and reference U.S. Pat. No. 4,613,811 provides an optical cable disposed between two components--one of which compensates for temperature, loop degradation, and linearity.